Gamma rays detected from Cygnus?X-1 with likely jet origin

摘要

Aims. We probe the high-energy ( & 60?MeV) emission from the black hole X-ray binary system, Cygnus?X-1, and investigate its origin. Methods. We analyzed 7.5?yr of data by Fermi -LAT with the latest Pass 8 software version. Results. We report the detection of a signal at ~ 8 σ statistical significance that is spatially coincident with Cygnus?X-1 and has a luminosity of 5.5 × 10~(33) ?erg?s ~(-1) , above 60?MeV. The signal is correlated with the hard X-ray flux: the source is observed at high energies only during the hard X-ray spectral state, when the source is known to display persistent, relativistic radio-emitting jets. The energy spectrum, extending up to ~ 20?GeV without any sign of spectral break, is well fit by a power-law function with a photon index of 2.3 ± 0.2 . There is a hint of orbital flux variability, with high-energy emission mostly coming around the superior conjunction. Conclusions. We detected GeV emission from Cygnus?X-1 and probed that the emission is most likely associated with the relativistic jets. The evidence of flux orbital variability indicates the anisotropic inverse-Compton on stellar photons as the mechanism at work, thus constraining the emission region to a distance 10 ~(11) – 10~(13) ?cm from the black hole.